Retorting is a process commonly used to prepare foods as well as sterilize them or other materials such as medical supplies, etc. in their packaging prior to distribution; e.g., to cook the foods within the packaging and kill microorganisms such as bacteria in the materials as well. Retorting is a heating process comprising subjecting the packaged contents to heat such as from heated or boiling water, or steam at an elevated temperature typically about 212.degree. to 275.degree. F. Heated water above 212.degree. F. can be used in retorting with the appropriate pressure.
Pouches as packaging for retorting exist: see, e.g., U.S. Pat. Nos. 4,190,477, 4,311,742, 4,360,550 and 4,424,256. A feature common to these prior art pouches is a layer of metal foil. The foil is a gas barrier which maintains such properties even after the rigors of retorting.
Consumers use microwave ovens to prepare or reheat prepackaged foods. Likewise, industry may desire to employ microwave processes for preparing and sterilizing packaged materials. As is well known, metal objects generally are not to be used in microwave processes such as microwave oven food preparation, reheating or microwave pasteurization. Thus, packaging having a foil layer is ill-suited to many of the needs of today.
For instance, food prepared, packaged and sold for preparation/reheating in the package by the consumer may not be able to meet the needs of the consumer if he cannot employ a microwave oven to prepare/reheat the food for consumption because the package has a foil layer. Furthermore, if such prepackaged food cannot be subjected to microwave preparation/reheating, it simply may not be desirable to the consumer having a microwave oven and desiring to enjoy its benefits in preparation/reheating food, e.g., speed of preparation, less generation of radiant heat, etc. Likewise, commercial purchasers may object to packaging for their products if the packaged products cannot be subjected to microwave pasteurization.
Thus there is a great demand for packaging which does not have a foil layer.
However, retorting and microwave processes subject packaging to extreme conditions: high temperature; pressurized and/or heated water or steam, etc. With respect to microwave processes, it is noted that water in a packaged material may be converted to steam, thereby subjecting the packaging to conditions akin to retorting conditions. Therefore, there is a great demand for packaging which can withstand the rigors of retorting conditions while not having a foil layer.
Polyvinylidene chloride copolymers are supplied as resins, e.g., Dow Chemical markets resins under the mark Saran. These Saran resins are known as VC Saran and MA Saran. Dow manufactures single layer VC Saran film under the name HB Saran. MA Saran is a methyl acrylatepolyvinylidene chloride copolymer. For reference, mention is made of WO 89/03411 (International Application No. PCT/US 88/03515; Priority Application U.S. application Ser. No. 107,141, filed Oct. 9, 1987) and WO 89/3412 (International Application No. PCT/US88/03516; Priority Application U.S. application Ser. No. 107,137, filed Oct. 9, 1987), each of which being hereby incorporated herein by reference. These International Applications not only relate to polyvinylidene chloride copolymers and compositions thereof, but, these documents evince the current thinking with respect to such copolymers and compositions.
in these International Applications, it is recognized that films or other articles can be formed from polyvinylidene chloride copolymers and that these articles "should" have low-gas permeabilities to, for example, oxygen, carbon dioxide, water vapor, odor bodies, flavor bodies, hydrocarbons or agricultural chemicals.
However, to obtain such articles, the resins of the polyvinylidene chloride copolymers must be extruded. Therein lies a problem. When no modifiers are used with the resins, the melt viscosity of the resins is so high and the load on the extruder screw is so large such that the copolymers are subject to thermal degradation. The decomposed copolymers may generate undesirable levels of carbon and hydrochloric acid in the extrudate.
Moreover, as recognized in the aforementioned International Applications, in order to industrially extrude resins of polyvinylidene chloride copolymers without thermal decomposition and discoloring "a relatively large amount of a stabilizer and a plasticizer would inevitably" be incorporated into the resins.
In the art, it has been taught that ethylene vinyl acetate (EVA) can be added to resins of polyvinylidene chloride copolymers to improve extrudability without a significant concommitant loss in barrier properties. In fact, EVA-containing extrusion-aiding additives for polyvinylidene chloride copolymer formulations have been preferred by the art. Furthermore the art has equated EVA with other additive materials such as epoxidized oils in regard to EVA's plasticizer and lubricant characteristics. However, neither the aforementioned International Applications nor other publications relating to uses of polyvinylidene chloride copolymer extrudates, see, e.g., U.S. Pat. No. 4,714,638, recognize that retorting conditions can adversely affect the barrier properties of polyvinylidene chloride copolymer extrudates.
It has now been found that many packaging materials, particularly polymeric films suffer deterioration of their desirable properties such as gas barrier properties when subjected to retorting conditions. After retorting, the packaging materials become more permeable to gases such as oxygen and air, leading to a decreased shelf life for the materials within the package.
Ethylene-vinyl alcohol (EVOH) has been employed as an oxygen barrier in formulations for packaging materials. However, EVOH is moisture sensitive. It absorbs water during retorting and like processes and this reduces its oxygen barrier properties. With the passage of time and the loss of moisture, there is some barrier recovery. However, the packaged materials may already have suffered oxidative spoilage.
Films from the Saran resins have been known to be good gas barriers, unaffected by the retort process. Or, the art simply did not recognize that retorting conditions adversely affected films, particularly certain films as discussed below, from the Saran resins. It has been found, however, that as a result of retorting, HB Saran films suffer from an increase in oxygen permeability of at least approximately 30-70%. This increase in oxygen permeability is observed even though the HB Saran does not contain EVA. Similarly, films from MA Saran having EVA as an extrusion-aiding additive and other films made with methyl acrylate-polyvinylidene chloride copolymers blended with ethylene vinyl acetate suffer from an increase in permeability of at least approximately 30-100% as a result of retorting.
It is a principal object of this invention, therefore, to provide novel methyl acrylate-polyvinylidene copolymer compositions suitable for forming novel gas barrier films which will maintain their gas barrier properties under conditions of severe stress such as retorting.